An x-ray tomography system can provide an image of the internal structure of a sample without having to destroy or cross-section the sample. X-rays produced by the system are passed through the sample and detected by an x-ray detector to obtain an absorption image from a cross-section of the sample. The x-ray detector can be 2 dimensional, in which case multiple cross-sectional images can be obtained at the same time. The sample and or x-ray source and detector are incrementally rotated, and additional cross sectional images are obtained from different angles. Multiple cross-sectional images obtained in this manner are mathematically manipulated to obtain sample information to reconstruct an image of the interior of the sample.
Two important parameters of an x-ray tomography system are its resolution and its throughput. The resolution refers to how small a feature the system can image, and the throughput refers to how fast the system can acquire an image. Throughput can be increased by increasing the flux of x-rays passed through the sample, however, this typically decreases the resolution. X-ray tomography system designs are therefore often a compromise between throughput and resolution. While some high resolution systems have been described in academic literature, they typically require an undesirably long image acquisition time.
In commercial x-ray tomography systems, x-rays are typically generated by directing a high energy beam of electrons toward a target. As the electrons come to rest, they produce x-rays known as bremsstrahlung having frequencies that lie along a continuous frequency spectrum. In addition, some of the electrons collide with and eject electrons in the inner shells of the target atoms. The vacancies created by these ejected electrons are subsequently filled by electrons in the outer shells of the target atoms, which drop in energy level by spontaneously emitting characteristic x-rays whose energies are determined by the differences between the energy levels of the inner and outer shell electrons. Both types of x-rays can contribute to x-ray imaging, however the flux of characteristic x-rays is typically much larger than the flux of bremsstrahlung, and so characteristic or hard x-rays typically contribute more to the x-ray absorption images.
The resolution of an x-ray tomography system without x-ray focusing optics is determined in large part by the effective size of its x-ray source. For systems that use an electron beam to generate x-rays, the effective source size is determined by the volume within which the beam electrons interact with and come to rest in the target. This interaction volume is largely determined by the density and atomic number of the target material, and the diameter and energy of the electron beam, and is typically tear-drop shaped.
An x-ray source for a typical x-ray tomography system 100 is shown in FIG. 1. The source consists of an electron beam 105, and a target 120. The target is typically made by depositing a thin metal film 130 of high atomic number and density (e.g., tungsten) on a substrate 140 of low atomic number and density (e.g., silicon). The target is typically tilted at an angle 150 of about 45 degrees with respect to the electron beam 105. Increasing the energy of electron beam 105 increases the interaction volume within x-ray target 120 (e.g., from a smaller interaction volume 160 to a larger interaction volume 170), thereby increasing the flux of x-rays produced in the target and the throughput of the x-ray tomography system. However, increasing the electron beam energy also increases the effective source size of the x-ray target (e.g., from a smaller effective source size 165 to a larger effective source size 175), thereby decreasing the resolution of the x-ray tomography system.
In some x-ray tomography systems, x-ray optics are used to focus the x-rays produced in the target to reduce the effective source size. However, x-ray optics absorb some of the incoming x-ray flux and typically have a limited depth of focus. As a result, portions of a sample that are not in the focal plane of the x-ray beam but that contribute to the image of the sample tend to decrease the resolution, thereby at least partially offsetting the resolution gains made by focusing the x-ray beam. Moreover, x-ray optics add additional system expense and complexity, including the need to properly align the optical system.
Stand-alone x-ray tomography systems are relatively expensive with prices greater than a million dollars. A much less expensive option is to add a metal target, rotating sample stage, and x-ray detector to a scanning electron microscope (SEM). The electron beam of the SEM can be focused onto the metal target to generate x-rays, which subsequently pass through a sample mounted on the sample stage to an x-ray detector to obtain an absorption image. The absorption images are typically obtained in a projection mode, with the sample positioned between the x-ray source (target) and the x-ray detector. The x-ray flux produced by the electron beam is dependent on the beam energy and the beam current. Because the focusing columns of most SEMs are primarily designed for forming secondary electron images, the electron beam current is typically limited to less than 75 nA and the beam energy is typically limited to 30 keV. The resultant x-ray flux produced by the electron beam of a typical SEM is therefore relatively low, and these systems require relatively long image acquisition times and have limited resolution.
Sasov et al., in “New type of x-ray source for lens-less laboratory nano-CT with 50 nm resolution,” Developments in X-ray Tomography VII, Proc. of SPIE Vol. 7804, describes one way to reduce the interaction volume and therefore the effective source size of an x-ray target. Sasov uses a hair-like tip of a metal wire as a target. The tip has its axis pointed in the direction of the detector, which increases the depth from which x-rays are generated, but does not greatly increase the width, thereby increasing the x-ray flux without increasing the effective source size. The x-ray flux generated from a small diameter rod-shaped target, however, is still relatively low, and so image acquisition time would still be long. Sasov et al. does not state an image acquisition time. Sasov's x-ray source also suffers from lack of a heat sink. As the energy and/or flux of electrons used to generate x-rays increases, Sasov's source lacks a mechanism for dissipating the extra heat thereby generated.
Cazaux et al., in “Recent developments in X-ray projection microscopy and X-ray microtomography applied to materials science,” Journal de Physique IV, Colloque C7, supplement au Journal de Physique 111, Vol. 3, November 1993, pp. 2099-2104, describes a system in which a target is impacted by an electron beam, and the x-rays produced are transmitted through the target and out of a vacuum chamber toward a sample and a detector. Cazaux' system also allows the target generating the incoming x-ray beam to be changed in a few seconds, allowing different images of the same specimen to be obtained with different characteristic x-rays.